ProcessWire Page IDs Fieldtype
Field that stores Page IDs as simple, numeric values. Long story short: this Fieldtype was built as a workaround since Page Reference fields can't handle self-references due to ProcessWire's built-in circular reference prevention.
Fieldtype Page IDs is intended to be loosely compatible with Page References in order to make conversions between the two feasible, but on the other hand it is very much limited feature wise: advanced selectors are not supported, page values can't be directly accessed, only a couple of inputfields are currently supported, and so on.
Note: this module requires ProcessWire >= 3.0.98.
Clone or download the module's directory to /site/modules/, go to Admin > Modules, hit "Check for new modules", and install Fieldtype Page IDs.
Since this is a fieldtype module, the next step would be creating a field and choosing Page IDs as the fieldtype. In field configuration you should input a selector string to populate the values, and choose the inputfield from a list of supported inputfield types (default is AsmSelect).
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
(See included LICENSE file for full license text.)